AR# 3302

CPLD XC9500 Family TAEngine - What are negative setup times in CPLD Performance report?


Keywords: M1, Timing, CPLD, Negative, Setup time.

Urgency: Standard

General Description:
After running M1 and creating a 'Post Layout Timing Report',
in the timing report (a.k.a. Performance Summary Report),
there may be negative values for setup times such as below:

Setup to Clock at the Pad (tSU) : -13.0ns (0 macrocell levels)
Data signal 'CS10' to TFF D input Pin at '&__A__55.D'
Clock pad 'R_W' (Global Clock)

Negative setup time is the result of a clock signal that is
slower than the total delay and setup on the data input to
the ff. Normal (positive) setup time is when the ff data
must be stable before clock comes. The equation to calculate
the time is:


In the case where setup time is negative, the data arrives
at the ff, stays for setup, then disappears before the clock
comes. In the above equation, Tgck is larger than the sum of
the rest, therefore there is a negative Tsu.

For more information on this or equations for other configurations,
please see the Xilinx App note XAPP071 - Using the XC9500 Timing


To resolve the timing problem, the data must be maintained on the
input of the ff until clock comes (Tgck). The negative Tsu is the
additional time the data must remain valid before the clock comes.

AR# 3302
Date 06/13/2002
Status Archive
Type General Article